The technical field of this invention is chemical synthesis and, in particular, synthesis of silica aerogels by sol-gel processes, and aerogel compositions made by these techniques.
Silica aerogels have attracted the attention of researchers in various fields of science and technology. Silica aerogels have been referred as “frozen smoke” by their nebulous appearance. Silica aerogels will appear yellowish if a light source is viewed through them, and appear light blue under sunlight. Both phenomena are due to Rayleigh scattering, which results from micro porous structures. Typical silica aerogels consist of nano-sized open pores with extremely low densities (0.003-0.35 g/cm3) and large surface area (200-1600 m2/g). As a result of their unique microstructure, silica aerogels exhibit many fascinating physical properties, such as extremely low thermal conductivity, low acoustic velocity, low dielectric constant, tunable refractive index, etc.
The first application of silica aerogel was reported for the use in Cerenkov detectors based on the ability of tunable refractive index from 1.01 to 1.2 and this essentially helps to replace the use of compressed gas as the detection media. Another interesting application of silica aerogels is for acoustic impedance matching devices due to their various and rather low acoustic impedance values from different combinations of sound propagation velocities and densities. Yet, another promising application of silica aerogels is their use in thermal insulations. Transparent or translucent aerogels can be used as spacers in windows for day light applications and improve the use of solar energy, since the aerogels insulation is very effective. The non-transparent aerogels have complementary applications where transparency is not required, such as refrigerators, heat storage devices and building insulations.
The process to produce silica aerogels is called sol-gel process. In a sol-gel process, a solution of silicate precursors undergoes changing from monomers to sol colloidal particles, which cross link into three-dimensional networks. It has been known that the structure of gel network and the physical properties of silica aerogels strongly depend on the preparation of precursors' solutions and the chemical reaction conditions during sol-gel process. Silica aerogels of the present invention are produced from sol-gel process based on either 1-step acid or basic method, or 2-step acid-basic method. Because of the dependency of the hydrolysis and condensation reaction on the pH, sol particles will grow through derived models called reaction limited cluster aggregation (RLCA) under acid condition, or reaction limited monomer cluster growth (RLMC) under basic condition. Usually, polymeric-like network with small pores is formed under acid condition owing to the entanglement of less branched long chains, while, under basic condition, highly condensed structure with larger pores is formed from the aggregation of larger clusters. With the 2-step sol-gel method, a more deliberate control of the network structure has been realized, and production of silica aerogels with lower density and better transparency becomes possible.
A distinctive characteristic property of silica aerogels is the brittleness, resulting from their complex microstructure, so the compressive/tensile strength and elastic modulus of silica aerogels are very low. It is believed that the mechanical properties of silica aerogels are strongly dependent on the degree of network connectivity resulting from various sol-gel processing conditions. For example, it has been reported that silica aerogels prepared under acid or neutral catalytic conditions will appear twice stiffer than the aerogels prepared under base catalytic condition. Aging treatment after the gelation can change the gel network strength thus making gels more sustainable to the capillary stresses, and improving their mechanical properties. Recent investigations have also shown that organically modified aerogels can have improved elastic properties. However, there still exists a need for improvements to the mechanical properties of silica aerogels to make them sufficiently strong and robust for many desired applications.
The most attracting property of silica aerogels is their extraordinarily low thermal conductivities. The total thermal conductivity of silica aerogels consists of three components: solid conduction, gas conduction and radiation. The solid conduction will increase with increasing density, while gas and radiation transports will decrease. In order to further minimize the thermal conductivity, evacuated silica aerogels have been investigated and a thermal conductivity of 0.010 W/m·K has been reported, comparing to 0.020 W/m·K with air. Another approach is to reduce the radiation transport with addition infrared opacifiers, such as carbon. At ambient pressure, the addition of carbon could lower the thermal conductivity to 0.0135 W/m·K and ˜0.0042 W/m·K under vacuum condition.
To date, researchers only have some fundamental understandings of the chemistry-structure-properties relationships of silica aerogels, however, most aerogels are still prepared empirically, and it is far away from chemically designed aerogel properties. Thus, the need for in depth understanding and better control of the chemical processes during gel network formation is needed. Furthermore, to promote and extend silica aerogels' applications, it is necessary to reinforce the mechanical properties of silica aerogels while retaining their fascinating properties, especially low thermal conductivities.